1. Field of the Art
The present invention relates generally to a method of manufacturing a lobe-type rotor of a rotary fluid machine of a Roots type, and more particularly to a method of broaching the outer surface of the rotor to a desired finish profile, and a broach unit or cutter used in the broaching method.
2. Brief Discussion of Related Art
In a rotary fluid machine of a Roots type such as rotary lobe-type pump or air compressor or blower, two lobes or rotors mounted on parallel shafts are received in a hollow housing and are operatively connected to each other by timing gears located at one end of each parallel shaft, so that the two rotors are rotated in opposite directions with a proper relation maintained therebetween. Generally, it is important to minimize the clearance or gap between the peripheries of the lobe-type rotors, for maximum discharge or displacement efficiency. To this end, it is desirable that an outer surface of a resin layer covering the external surface of the core member of each rotor be accurately finished to the intended profile. To finish the outer periphery of the resin-coated rotor with high efficiency, a broaching method is commonly practiced to profile the outer surface of the resin layer over a relatively large area in a single pass of a broach having a formed cutting edge corresponding to the surface to be broached. In this case, however, a chip produced by the formed broach cutter tends to be continuous, causing a comparatively high cutting resistance to the broach cutter, and consequent inconveniences such as local or partial separation, flake-off or peel-off of the resin layer from the core member of the rotor, and/or breakage or other damage to the broach cutter.
To reduce the cutting resistance, it has been proposed to use a combination or composite broach unit as indicated generally at 18 in FIG. 14. This broach unit includes a slide 10, and two cutters 14, 16 which are mounted on the slide 10 such that the cutters 14, 16 are spaced apart from each other in a direction of movement of the slide 10, i.e., in the direction parallel to the axis of rotation of a lobe-type rotor 12 to be finished by the broach unit 18. The two cutters 14, 16 have cutting edges whose contours cooperate to define a portion of the outer periphery of the rotor to be broached. Further, the two cutters 14, 16 have a plurality of chip-breaker recesses 20, 21 which are formed in the broaching direction and spaced apart from each other along the surface of the rotor 12 to be broached. Each recess 21 is disposed between the adjacent two recesses 20, in the direction along the surface of the rotor 12.
The above broaching method utilizing two formed cutters is advantageous in terms of ease of chip removal, owing to the presence of the chip-breaker recesses 20, 21, which function is to break the produced chip into small segments and thereby eliminate the possibility of flake-off or peel-off of the resin layer of the rotor 12. In the proposed broaching method, projections 22 are left uncut, as shown in FIG. 15, due to the presence of the chip-breaker recesses 20 in the lower or leading cutter 14. These projections 22 which extend in the broaching direction, are subsequently removed by the upper or trailing cutter 16, as indicated in FIG. 16. However, the projections 22 are left unremoved as raised portions 24, due to a slight misalignment of the lower and upper cutters 14, 16 relative to each other, or due to a difference in the amount of grinding for sharpening the cutting edges of the two cutters 14, 16. Further, raised portions 25 may be left due to the presence of the chip-breaker recesses 21 formed in the upper cutter 16, as indicated in FIG. 17. Thus, the proposed broaching method and composite broach unit are characterized by unsatisfactory smoothness of the finished surface of the rotor 12. The above-indicated inconveniences become increasingly severe as the broaching area of the cutters 14, 16 is increased, and as the cutting edge profiles of the cutters 14, 16 become complicated.